Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.269n&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 269
Other
Scientific paper
Cosmic spherules and fragments in the size range 0.1-1 mm, originally extracted magnetically from deep-sea deposits, have now been isolated from ice in Greenland and Antarctica (Maurette et al., 1986; Koeberl and Hagen, 1989; Harvey and Maurette, 1990). Studies of cosmogenic radionuclides in individual spherules made possible by accelerator mass spectrometry (Raisbeck et al., 1985; Nishiizumi et al., 1991, 1992), have verified the model calculations that suggested that the spherules are mainly fused micrometeoroids, rather than spall droplets from larger objects (Dohnanyi, 1978; Grun et al., 1985; Olinger et al., 1990). However, the exposure lifetimes observed are much longer than those deduced from models (10^5- 10^7 years as against a few times 10^4) (Dohnanyi, 1978; Grun et al., 1985). As new sources of spherules become available, it is important to verify their extraterrestrial origin, and to see whether they confirm or modify earlier findings. It is also interesting to continue to search for spall droplets. In this paper we report on a group of large spherules collected from glacial till near Lewis Cliff, Antarctica (84.3 degrees S, 161.6 degrees E) (Harvey and Maurette, 1990). The concentrations of cosmogenic ^10Be and ^26Al were measured in 10 such spherules (LC-7 to LC-16). Typically these spherules have a lower Fe content, and are more weakly magnetic, than those studied earlier. In addition to these spherules, we analyzed 8 individual deep sea spherules (90-250 micrograms) (Murrell et al., 1980). All particles were individually mounted in acrylic resin and a small surface was polished flat with aluminum oxide. The quantitative elemental analysis of these polished surfaces was performed using an electron microprobe. After electron microprobe analysis, each particle was dissolved and Be and Al were separated for AMS. The ^10Be and ^26Al concentrations were determined at LLNL. In this and a preceding group of LC and deep-sea particles, three (LC-6, reported previously by Nishiizumi et al., 1992, LC- 10 and LC-15) have contents of ^10Be and ^26Al that are not inconsistent with their being spall droplets from larger objects, bringing the number of possible cases to three out of 42 stony objects so far reported. In all other cases, clear evidence of SCR bombardment (^26Al/^10Be greater than the ratio produced in meteorites by GCR) is present. An earlier paper (Nishiizumi et al., 1991) discusses two acceptable models (4-pi and 2-pi) for the bombardment history of the particles bearing a clear SCR signature. The present data strengthen our preference for the 4-pi model; measurements of other nuclides will provide further constraints. References Dohnanyi J. S. (1978) in Cosmic Dust (ed. J. A. M. McDonnell) 527-605, (J. Wiley). Grun E. et al. (1985) Icarus 62, 244-272. Harvey R. P. and Maurette M. (1990) Lunar Planet. Sci. XXI, 467- 468. Koeberl C. and Hagen E. H. (1989) Geochim. Cosmochim. Acta 53, 937-944. Maurette M. et al. (1986) Science 233, 869-872. Murrell M. T. et al. (1980) Geochim. Cosmochim. Acta 44, 2067- 2074. Nishiizumi K. et al. (1991) Earth Planet. Sci. Lett. 104, 315-324. Nishiizumi K. et al. (1992) Lunar Planet. Sci. XXIII, 997-998. Olinger C. T. et al. (1990) Earth Planet. Sci. Lett. 100, 77-93. Raisbeck G. M. et al. (1985) in Properties and Interactions of Interplanetary Dust (eds. Giese R. H. & Lamy P.) 169-174 (D. Reidel).
Arnold James R.
Brownlee Don E.
Chaffee M. W.
Finkel Robert C.
Harvey Ralph P.
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